High entropy alloy, method of preparation and use of the same

ABSTRACT

A high entropy alloy includes at least five elements selected from Cobalt, Nickel, Titanium, Zirconium, and Hafnium, wherein two of the five elements have a total atomic percentage of 100−x, and the remainder elements have a total atomic percentage of x, where 0&lt;x&lt;100. A method of producing the high entropy alloy. A component for use in a mechanical timepiece. The component is made of the high entropy alloy.

TECHNICAL FIELD

The present invention relates to a high entropy alloy in particular ahigh entropy alloy comprising at least five elements. The presentinvention also pertains to a method of preparing and use of said alloy.

BACKGROUND OF THE INVENTION

It is appreciated that metallic materials is particularly important invarious industrial applications, including but not limiting toactuators, medical devices, and high precision instruments. Severalmetallic materials may be generally used for the aforementionedapplications, yet each of them possesses different kinds of defects.

For example, bulk crystalline metals plastically deform throughdislocations, twinning and/or grain boundary sliding, which limit theelastic strain of bulk crystalline metals to ˜0.2% at room temperature.The use of bulk amorphous alloys in industrial applications may behindered by the high cooling rate requirement which severely limits thesize of the alloys to be produced. On the other hand, whilst shapememory alloys and gum metals may be capable of achieving a large elasticstrain limit (˜2%), such a high strain limit is generally associatedwith significant mechanical hysteresis and energy dissipation.

Accordingly, there remains a strong need for developing metallicmaterials such as alloy materials that are capable of exhibiting linearelastic response to very high strains without hysteresis, and elasticproperties which are temperature-insensitive (to high temperature).

SUMMARY OF THE INVENTION

The first aspect of the present invention relates to a high entropyalloy comprising at least five elements selected from Cobalt, Nickel,Titanium, Zirconium, and Hafnium, wherein two of the five elements havea total atomic percentage of 100−x, and the remainder elements have atotal atomic percentage of x, where 0<x<100.

Advantageously, the inventors have first devised that by tuning thechemical composition of the high entropy alloy, it may engineer thedisorder of the high entropy alloy, thereby providing a new route tocreate temperature-independent, ultra-elastic behavior in a wide rangeof materials.

In particular, the high entropy alloy is represented by a chemicalformula of (CoNi)_(100−x)(TiZrHf)_(x), where x is an atomic percentageand 0<x<100.

Preferably, the high entropy alloy is represented by a chemical formulaof (CoNi)_(100−x)(TiZrHf)_(x), where 45≤x≤55.

In an embodiment, the high entropy alloy has an elastic module that issubstantially constant with respect to a temperature change from 300K to900K.

In another embodiment, the high entropy alloy comprises a body centredcubic (BCC) structure. In particular, atoms of the high entropy alloyare accommodated within the BCC structure by atomic-scale chemicalordering.

In another embodiment, the high entropy alloy comprises a distortedlattice structure.

In another embodiment, the high entropy alloy has an atomic sizedifference of about 11%.

In another embodiment, the high entropy alloy has an elastic limit ofabout 2%.

The present invention in another aspect provides a method of producing ahigh entropy alloy as described above. The method comprises the stepsof:

-   -   preparing an alloy precursor by arc melting a predetermined        amount of raw materials of each elements constituting the high        entropy alloy in an inert atmosphere; and    -   casting the melted alloy precursor into a cooled mold to obtain        the high entropy alloy.

In an embodiment, the raw materials comprises Cobalt, Nickel, Titanium,Zirconium, and Hafnium. In particular, the raw materials are in atomicpercentages of: 0-50% Cobalt, 0-50% Nickel, 0-33.3% Titanium, 0-33.3%Zirconium, and 0-33.3% Hafnium.

In another embodiment, the raw materials have a purity of >99.9%.

In another embodiment, the method further comprises the step of flippingand remelting the alloy precursor in a repetitive manner.

In another embodiment, the arc melting is conducted under a Ti-getteredargon atmosphere with a pressure below 8×10⁻⁴ Pa.

In another embodiment, the mold is a copper mold. In particular, thecopper mold is a cylinder or a plate.

In another embodiment, the cylindrical copper mold has a diameter of 5mm and a length of 100 mm.

In another embodiment, the plate copper mold has a dimension of 5×10×60mm³.

Further provided with the present invention is a component made of thehigh entropy alloy as described above for use in a mechanical timepiece.In particular, the component is a mainspring and/or a hairspring.

In an embodiment, the mechanical timepiece is selected from the listcomprising mechanical watches, mechanical chronometers, and marinechronometers.

Those skilled in the art will appreciate that the invention describedherein is susceptible to variations and modifications other than thosespecifically described. The invention includes all such variations andmodifications. The invention also includes all steps and featuresreferred to or indicated in the specification, individually orcollectively, and any and all combinations of the steps or features.

Other features and aspects of the invention will become apparent byconsideration of the following detailed description and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot showing the X-ray Diffraction (XRD) patterns ofdifferent alloys having a chemical formula of(CoNi)_(100−x)(TiZrHf)_(x).

FIG. 2 is a bar chart showing the microhardness of different alloyshaving a chemical formula of (CoNi)_(100−x)(TiZrHf)_(x).

FIG. 3 is a bar chart showing the Young's modulus of different alloyshaving a chemical formula of (CoNi)_(100−x)(TiZrHf)_(x).

FIG. 4 is a plot showing the storage modulus against differenttemperature of different alloys having a chemical formula of(CoNi)_(100−x)(TiZrHf)_(x).

DETAILED DESCRIPTION OF THE INVENTION

Unless otherwise defined, all technical terms used herein have the samemeaning as commonly understood by one skilled in the art to which theinvention belongs.

As used herein, “comprising” means including the following elements butnot excluding others. “Essentially consisting of” means that thematerial consists of the respective element along with usually andunavoidable impurities such as side products and components usuallyresulting from the respective preparation or method for obtaining thematerial such as traces of further components or solvents. Theexpression that a material is certain element is to be understood formeaning “essentially consists of” said element. As used herein, theforms “a,” “an,” and “the,” are intended to include the singular andplural forms unless the context clearly indicates otherwise.

It is intended that reference to a range of numbers disclosed herein(for example, 1 to 10) also incorporates reference to all rationalnumbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5,7, 8, 9 and 10) and also any range of rational numbers within that range(for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, allsub-ranges of all ranges expressly disclosed herein are hereby expresslydisclosed. These are only examples of what is specifically intended andall possible combinations of numerical values between the lowest valueand the highest value enumerated are to be considered to be expresslystated in this application in a similar manner.

In general, it is appreciated that the formation of concentratedmulti-component alloys (i.e. high entropy alloys, HEAs) requiresminimization of overall atomic size misfit in these alloys so as to formsingle-phase solid solutions, or otherwise it would favors the formationof amorphous phase structure.

Without wishing to be bound by theories, the inventors have, throughtheir own research, trials, and experiments, devised a high entropyalloy in which the atomic size misfit may be considered to be too largein view of the conventional solid solution alloy design rules. The highentropy alloy of the present invention may also have an atomic-scaledistortions within a single crystalline phase, which renders the highentropy alloy an ultra-large elastic strain limit and negligibleinternal friction at room temperature, and substantially constantelastic properties up to 900K.

The high entropy alloy provided by the present invention comprises atleast five elements selected from Cobalt, Nickel, Titanium, Zirconium,and Hafnium. In particular, two of the five elements have a total atomicpercentage of 100−x, and the remainder elements have a total atomicpercentage of x, where 0<x<100. In other words, the high entropy alloymay be represented by a chemical formula of (AB)_(100−x)(CDE)_(x), whereA, B, C, D, and E are not identical and each of which is selected fromCobalt, Nickel, Titanium, Zirconium, and Hafnium.

In one example, the high entropy alloy may be represented by a chemicalformula of (CoNi)_(100−x)(TiZrHf)_(x), where x is an atomic percentageand 0<x<100. Preferably, the high entropy alloy may be represented by achemical formula of (CoNi)_(100−x)(TiZrHf)_(x), where 45≤x≤55.

In one example embodiment, the inventors have applied the followingequation (Eq. 1) and devised that the high entropy alloy of the presentinvention may have an atomic size difference that is considered to betoo large for forming a stable, single phase structure:

Delta=√{square root over (Σ_(i=1) ^(N) c _(i)(1−r _(i)√{square root over(r)})²)}  (Eq. 1),

where N is the number of constituent elements in the alloy, c_(i) theatomic fraction of the i-th component, r_(i) the atomic radius of thei-th component and r the average atomic radius; the atomic radii of theconstituent elements are 1.251 Å for Co, 1.246 Å for Ni, 1.578 Å for Hf,1.462 Å for Ti and 1.603 Å for Zr.

For example, the value of x may be equal to 50, i.e. the high entropyalloy may have a chemical formula of (CoNi)₅₀(TiZrHf)₅₀, the atomic sizedifference may be calculated to be about 11%. It is appreciated that ingeneral the Delta value may be of 5 or even less in conventional solidsolution alloy design rules/theories. The atomic size difference of thehigh entropy alloy in the present disclosure would therefore have beenconsidered significantly large and such a large atomic size differencewould have destabilized the crystalline structure of the as-formed HEAs,leading to either phase separation or the formation of a single,amorphous structure.

Unexpectedly, the inventors found that the high entropy alloy of thepresent invention generally comprises a stable, single phase structure.With reference to FIG. 1, there is provided with a plot showing theX-ray diffraction (XRD) patterns of various alloys. As shown, all thehigh entropy alloys of (CoNi)₄₅(TiZrHf)₅₅, (CoNi)₅₀(TiZrHf)₅₀, and(CoNi)₅₅(TiZrHf)₄₅ possess a single phase structure without anysubstantial phase separation as compared with the conventional alloys of(CoNi)₁₀₀ and (TiZrHf)₁₀₀, which have a single phase face centred cubic(FCC) and a single phase hexagonal closely packed (HCP) structure,respectively. In particular, the high entropy alloys have a single phasebody centred cubic (BCC) structure, particularly an ordered BCC-likestructure, i.e. a B2 structure.

Within the BCC structure, atoms of the high entropy alloy may beaccommodated by atomic-scale chemical ordering. In one example, theatoms may be accommodated within the BCC structure in an ordered manner.For example, sites in sublattice A of the B2 structure (corner sites ofBCC unit cell) may be occupied at random with {Co, Ni} and sites insublattice B (center sites of the BCC unit cell) may be occupied atrandom with {Hf, Ti, Zr}. In another example, the atoms may beaccommodated in a partially ordered manner such as with 25% of Zr atomsfrom sublattice A exchanging with Co and Ni atoms on sublattice B.

In addition to the atomic-scale chemical ordering, the high entropyalloy of the present invention may have a distorted lattice structure.Namely, the structure of the high entropy alloy may have a combinationof atomic-scale chemical ordering and lattice distortion.

The combination of atomic-scale chemical ordering and lattice distortionwithin the single phase structure of the high entropy alloy may beadvantageous for said high entropy alloy to offer outstanding mechanicalproperties as compared with conventional alloys and/or HEAs withminimized atomic size difference. For example, with reference to FIGS.2-4, there are provided with a plurality of plots illustrating themicrohardness and Young's modulus of the different alloys. As shown inFIG. 2, all the high entropy alloys possess a microhardness of at least480 HV, which is apparently higher than those of the conventional alloys(CoNi)₁₀₀ and (TiZrHf)₁₀₀. In particular, the microhardness of(CoNi)₄₅(TiZrHf)₅₅ is about 800 HV, which is the highest among the threehigh entropy alloys.

Turning now to FIG. 3, there is provided with a plot illustrating theYoung's modulus of the different alloys at room temperature. The term“Young's Modulus” generally refers as “elastic modulus” which defines amaterial's resistance to non-permanent, or elastic, deformation.“Elastic deformation” generally refers to the change of physical statesuch as shape of the material when an external force is applied thereto,and the material will return to its original state after the stress isremoved. It is appreciated that once a material experiences an externalforce that could overcome such resistance, the material will deformpermanently, i.e. the material no longer be able to return to itsoriginal state even the external force is removed. Namely, a materialwith a higher modulus would require a larger amount of external force toovercome such resistance, or in other words a material with a smallmodulus would deform permanently under such the same amount of externalforce. As shown in FIG. 3, the high entropy alloys of(CoNi)₄₅(TiZrHf)₅₅, (CoNi)₅₀(TiZrHf)₅₀, and (CoNi)₅₅(TiZrHf)₄₅ possess aYoung's modulus at room temperature ranging from about 100 to 130 GPa,which is comparable to the conventional alloys (CoNi)₁₀₀ and(TiZrHf)₁₀₀. That is, the high entropy alloys have a comparableresistance to non-permanent, or elastic, deformation as if theconventional alloys at room temperature.

In addition to the comparable Young's modulus at room temperature, it isadvantageous that the high entropy alloys may have an elastic modulethat is substantially constant with respect to a temperature change upto a temperature of at least 600 K, particularly at least 700 K,preferably at least 800 K, most preferably at least 900 K. For example,with reference to FIG. 4, there is provided a plot illustrating a changeof Young's modulus of different alloys against different temperatures.As shown, the Young's modulus of the conventional alloys decreasesgradually toward 0 as the temperature increases from 300 K to 900 K, inwhich (CoNi)₁₀₀ decreases sharply toward 0 at about 550 K whereas(TiZrHf)₁₀₀ decreases toward 0 at a constant rate with the temperatureincrease. In sharp contrast, all the high entropy alloys showed asubstantially constant Young's modulus when the temperature increasesfrom 300 K to 900 K. In other words, it may be considered that theelasticity of the high entropy alloys of the present invention remainssubstantially unchanged with respect to a wide range of temperature, orthe high entropy alloys have an Elinvar effect over a temperature of 300K to 900 K.

The present invention in another aspect provides a method of preparingthe high entropy alloy as described above. The method comprises thesteps of: preparing an alloy precursor by arc melting a predeterminedamount of raw materials of each elements constituting the high entropyalloy in an inert atmosphere; and casting the melted alloy precursorinto a cooled mold to obtain the high entropy alloy.

In one example, the alloy precursor may be prepared by providing the rawmaterials in atomic percentages of: 0-50% Cobalt, 0-50% Nickel, 0-33.3%Titanium, 0-33.3% Zirconium, and 0-33.3% Hafnium in an arc furnace. Theraw materials may be of a high purity such as >90%, particularly >95%,preferably >99%, most preferably >99.90%.

The aforementioned raw materials may be melted in an arc furnace underan inert atmosphere. Preferably, the arc furnace is pump-filled withTi-gettered argon gas, for example, 5 times such that the pressureinside the furnace is less than 8×10⁻⁴ Pa.

During the arc melting process, the raw materials may be flipped andremelted in a repetitive manner so as to ensure chemical homogeneity. Inother words, to ensure each of the raw material components are uniformlydistributed. Preferably, the raw materials are flipped and re-melted forat least five times.

Once the raw materials are completely arc melted, the resultantmaterial, that is the melted alloy precursor, may be casted into acooled mold to form the high entropy alloy. In particular, the meltedalloy precursor may be casted into a copper mold of different shapes anddimensions so as to obtain a high entropy alloy of desired shape anddimension. In one example, the melted alloy precursor may be casted intoa cylindrical copper mold having a diameter of 5 mm and a length of 100mm. In another example, the melted alloy precursor may be casted into aplate copper mold having a dimension of 5×10×60 mm³.

As mentioned above, the high entropy alloy of the present invention isadvantageous as its mechanical properties, particularly the elasticproperties remain substantially unchanged over a wide range oftemperature. This property may render the high entropy alloy of thepresent invention particularly suitable for use in components thatrequire to be operated in harsh environments. Thus, further providedwith the present invention is a component for use in a mechanicaltimepiece, where the component is made of the high entropy alloy asdescribed above.

In one example, the component may be a mainspring and/or a hairspring ofa mechanical timepiece. In particular, the mechanical timepiece may beselected from the list comprising mechanical watches, mechanicalchronometers, and marine chronometers. It is appreciated that mechanicaltimepieces are generally driven by a mainspring of which the force istransmitted through a series of gears to power the balance wheel(including a balance spring (i.e. hairspring)), a weighted wheel whichoscillates back and forth at a constant rate.

As temperature increases, it would significantly affect the timekeepingof the balance wheel and the balance spring as a result of decrease inYoung's modulus of the balance spring. Whilst it may be overcome byusing a temperature-compensated balance wheel, saidtemperature-compensated balance wheel is generally inoperable atextremes of temperature. Meanwhile, although some “auxiliarycompensation” mechanisms may be used to avoid this situation, all ofthem suffer from being complex and hard to adjust.

With the high entropy alloy of the present invention to be used in themechanical timepiece components, the temperature-insensitive elasticproperty of the high entropy alloy may allow the component to beoperated over a wide range of temperature without the need of“temperature compensation” even at an extreme temperature such as at 900K. Thus, advantageously, it may simplify the mechanical mechanism andtherefore the manufacturing process of the mechanical timepieces.

EXAMPLES Materials and Reagents Used

The polycrystalline samples of the high entropy alloy Co₂₅Ni₂₅(HfTiZr)₅₀(atomic %) were prepared through arc-melting in a high purity argonatmosphere. The purities of the raw materials for each element were atleast 99.9 wt. %. The ingots were remelted at least four times to ensurethe chemical homogeneity, and then suction cast into a copper mold. Twodifferent types of copper mold (rod and plate) were used. The dimensionof the cylindrical mold was 5 mm in diameter and 100 mm in length whilethat of the plate mold was 5×10×60 mm³. The single crystalCo₂₅Ni₂₅(HfTiZr)₅₀ alloys were prepared by a high rate directionalsolidification method following the standard procedure.

Instrumentation and Methods Applied

The X-ray diffraction (XRD) instrument (Rigaku Smartlab) was used toidentify the crystalline structure.

Dynamical mechanical analyses were performed in the commercial DMAequipment (Mettler Toledo, DMA1 STAR System). The experiments werecarried out by applying a sinusoidal stress at the fixed frequency of 1Hz during continuous heating at the constant heating rate of 5 K/min.The testing samples had a dimension of 30×3×1.5 mm³. The testing wasperformed in three-point bending mode.

Example 1 Synthesis and Characterization of Co₂₅Ni₂₅(HfTiZr)₅₀

The high entropy alloy Co₂₅Ni₂₅(HfTiZr)₅₀ (atomic %) (i.e.(CoNi)₅₀(HfTiZr)₅₀) was prepared via arc melting method. Unlikeconventional single phase HEAs, the atomic size difference ofCo₂₅Ni₂₅(HfTiZr)₅₀ alloy is extremely large, which is calculated to beabout 11% based on Eq.1, relative to standard approaches used for singlephase alloy design. It is appreciated that the established alloy phaserules/theories suggest that such a large atomic size misfit willdestabilize the crystalline structure, leading to either phaseseparation or the formation of a single phase, amorphous structure.X-ray diffraction (XRD) result demonstrated that the Co₂₅Ni₂₅(HfTiZr)₅₀high entropy alloy is a single-phase crystal with a nominallybody-centered cubic (BCC) like structure.

Example 2 Mechanical Properties of Co₂₅Ni₂₅(HfTiZr)₅₀

The mechanical properties were characterized by microhardness device.The hardness and young's modulus values of these alloys are presented inFIG. 2 and FIG. 3. The elastic strain limit of the Co₂₅Ni₂₅(HfTiZr)₅₀high entropy alloy is about 2%.

Example 3 Elinvar Effect of Co₂₅Ni₂₅(HfTiZr)₅₀

With reference to the FIG. 4, the elastic modulus of theCo₂₅Ni₂₅(HfTiZr)₅₀ alloy remained constant as the temperature increases,suggestive of an Elinvar effect (temperature-independent elasticconstants). Similarly, the elastic modulus of(CoNi)_(100−x)(HfTiZr)_(x), where x=45 and 55 are nearly constant withincreasing temperature.

1. A high entropy alloy comprising at least five elements selected fromCobalt, Nickel, Titanium, Zirconium, and Hafnium, wherein two of thefive elements have a total atomic percentage of 100−x, and the remainderelements have a total atomic percentage of x, where 0<x<100.
 2. The highentropy alloy of claim 1, wherein the high entropy alloy is representedby a chemical formula of (CoNi)_(100−x)(HfTiZr)_(x), where x is anatomic percentage and 0<x<100.
 3. The high entropy alloy of claim 1,wherein the high entropy alloy is represented by a chemical formula of(CoNi)_(100−x)(HfTiZr)_(x), where 45≤x≤55.
 4. The high entropy alloy ofclaim 1, wherein the high entropy alloy has an elastic module that issubstantially constant with respect to a temperature change from 300K to900K.
 5. The high entropy alloy of claim 1, wherein the high entropyalloy comprises a body centred cubic (BCC) structure.
 6. The highentropy alloy of claim 5, wherein atoms of the high entropy alloy areaccommodated within the BCC structure by atomic-scale chemical ordering.7. The high entropy alloy of claim 1, wherein the high entropy alloycomprises a distorted lattice structure.
 8. The high entropy alloy ofclaim 1, wherein the high entropy alloy has an atomic size difference ofabout 11%.
 9. The high entropy alloy of claim 1, wherein the highentropy alloy has an elastic limit of about 2%.
 10. A method ofpreparing the high entropy alloy of claim 1, comprising the steps of:preparing an alloy precursor by arc melting a predetermined amount ofraw materials of each elements constituting the high entropy alloy in aninert atmosphere; and casting the melted alloy precursor into a cooledmold to obtain the high entropy alloy.
 11. The method of claim 10,wherein the raw materials comprises Cobalt, Nickel, Titanium, Zirconium,and Hafnium.
 12. The method of claim 11, wherein the raw materials arein atomic percentages of: 0-50% Cobalt, 0-50% Nickel, 0-33.3% Titanium,0-33.3% Zirconium, and 0-33.3% Hafnium.
 13. The method of claim 10,wherein the raw materials have a purity of >99.9%.
 14. The method ofclaim 10, further comprising the step of flipping and remelting thealloy precursor in a repetitive manner.
 15. The method of claim 10,wherein the arc melting is conducted under a Ti-gettered argonatmosphere with a pressure below 8×10⁻⁴ Pa.
 16. The method of claim 10,wherein the mold is a copper mold.
 17. The method of claim 16, whereinthe copper mold is a cylinder or a plate.
 18. The method of claim 17,wherein the cylindrical copper mold has a diameter of 5 mm and a lengthof 100 mm.
 19. The method of claim 17, wherein the plate copper mold hasa dimension of 5×10×60 mm³.
 20. A component for use in a mechanicaltimepiece, wherein the component is made of the high entropy alloy ofclaim
 1. 21. The component of claim 20, wherein the component is amainspring and/or a hairspring.
 22. The component of claim 20, whereinthe mechanical timepiece is selected from the list comprising mechanicalwatches, mechanical chronometers, and marine chronometers.